Water-absorbing resins (SAP/Super Absorbent Polymer) are water-swelling and water-insoluble polymeric gelling agents, and are commonly used mainly in throwaway applications including sanitary goods such as disposable diapers and sanitary napkins, water retention agents for agriculture and horticulture, and industrial water sealants. As raw materials for these water-absorbing resins, numerous monomers and hydrophilic polymeric compounds are used. Among them, water-absorbing polyacrylic acid (salt) resins including acrylic acid and/or a salt thereof are most commonly used industrially because they have high water absorption performance.
The water-absorbing resin is produced as a particulate product through polymerization, drying, pulverization, classification, surface crosslinking and so on (Non-Patent Document 1), and a gas (exhaust gas) discharged particularly from each of the steps of polymerization, drying and heat treatment (surface crosslinking) should be caught before being released into the air from the viewpoint of environmental conservation. In other words, the exhaust gas contains water-absorbing resin raw materials and the like (monomer, crosslinking agent, water, organic solvent and so on), and these substances should be caught.
Thus, as techniques for catching or recycling the exhaust gas, there have been disclosed a technique in which discharges generated from a step of producing acrylic acid and polyacrylic acid are treated (Patent Document 1), a technique in which an exhaust gas removed from any stage in a step of producing a water-absorbing resin and an aqueous solution of a raw material of the water-absorbing resin is scrubbed with a basic aqueous solution (Patent Documents 2 to 4), and a technique in which vapors of water and a monomer generated in a polymerization step or the like are recycled to an aqueous monomer solution and the polymerization step (Patent Documents 5 to 11).
In the step of producing a water-absorbing resin, water is a widely used solvent for a monomer and a crosslinking agent, and a technique has been disclosed in which distilled water is used as a solvent during polymerization (Patent Documents 12 to 14).